The present invention relates to capacitors, more particularly to electrolytic capacitors, and still more particularly to wound, multiple anode, high energy density electrolytic capacitors.
A capacitor is a device used in electronic circuits to store electrical charge. Electrical charge Q, is measured in coulombs and one electron has a charge of about 1.6xc3x9710xe2x88x9219 coulombs. Typically, the electrical charge is stored on the surfaces of separated plates (electrodes) immersed in an electrolyte. The plates are generally layered and may be planar or wound, e.g. rolled, as for example in a spiral roll. A dielectric layer of mechanical separator material, such as dielectric paper and the like, is arranged to maintain the plates separated from physical contact. The charge creates an electrostatic field which exists between the plates and therefore creates a potential difference, or voltage V, between the plates.
Capacitance C, is measured in farads which is defined as one coulomb per volt. In general, the capacitance of a device is determined by dividing the charge stored on the plates by the voltage the charge creates across the plates. By increasing the capacitance, a greater charge can be stored per unit volt.
Generally, capacitance can be increased in two ways; by increasing the area of the plates and by reducing the separation distance between the plates, such as by using very thin dielectric separators. In an electrolytic capacitor, capacitance is achieved on the anode (+) plate by electrolytically forming a thin layer of dielectric oxide on the surface and immersing it in an electrolyte solution which functions as the negative (xe2x88x92) plate.
The energy, in joules, stored in a capacitor equals xc2xdCV2 wherein V is voltage. In some applications, it is desired to maximize the energy density of a capacitor package. One such application is in the biomedical arts, and especially in implantable devices such as medical defibrillators.
Defibrillators must be capable of supplying an intense burst of energy to the heart in a very short time. The battery power supply of a typical implantable defibrillator is incapable of producing this energy alone. Therefore, the battery is used to charge an electrolytic capacitor which is then used to deliver the energy to the heart. For obvious reasons, it is important to minimize the size of the capacitor which is usually the largest component in the defibrillator circuit. It is thus advantageous to use a capacitor having as high an energy density as possible. Wound configurations are generally highly reliable and more convenient to produce, but are subject to packaging inefficiencies.
One packaging inefficiency associated with wound capacitors is the presence of open space inherent in the use of winding mandrels. Wound capacitors utilize winding mandrels to tightly wrap the electrodes in a spiral, but upon withdrawal of the mandrel an open space is left in the center of the capacitor. The smaller the diameter of the mandrel the more anode material can be packaged in a defined cylindrical housing and the smaller the open space left from mandrel withdrawal.
Anode foil is generally available in two generic forms, a solid-core anode form wherein the core is considered effectively non-porous in nature and the surface is etched and generally comprises an oxide to maintain high voltage storage; and, a porous anode form which is generally deeply etched or otherwise treated to form pores through the foil, and which also generally comprises an oxide surface to maintain high voltage storage. Anode foils are generally brittle and tend to break when acutely bent, particularly when the bend is accompanied by tensile stress. Cathode foils are generally very thin flexible foils which bend easily without breaking.
Though small mandrel diameters are desirable for minimizing open spaces, practically speaking the minimum diameter of a commercially useful mandrel is limited by the brittleness of the anode foil. Breaking of an anode foil during assembly can cause both a disruption in the assembly process and a disruption in anode continuity, which can result in both lost manufacturing time and lost capacitor efficiency.
What is needed then, is a method of producing wound capacitors which permits reduction of mandrel diameter to decrease central open space and increase the overall capacitance efficiency of a cylindrical packaging housing.
One object of the invention is to provide a method for assembling a multiple anode capacitor which provides decreased open space within a cylindrical packaging container.
Another object of the invention is to provide a method for assembling a multiple anode capacitor which provides increased energy density within a defined container.
Still another object of the invention is to provide an extremely high energy density capacitor assembly especially suited for use in implantable medical devices such as defibrillators.
These and other objects of the invention will become apparent from the following recitation of the invention.
The present invention includes a multiple anode capacitor arrangement comprising a combination of porous and solid core anode foils, arranged to maximize capacitance which can be achieved in a defined volume, wound capacitor. More specifically, the invention comprises a multiple anode, aluminum electrolytic wound capacitor assembly comprising a cylindrical casing having closed ends and positive and negative terminal means. Disposed within the casing is a spiral wound capacitor body having a layered combination of porous and solid core anode aluminum foil layers, a cathode aluminum foil layer and one or more mechanical separator layers disposed between the cathode and the anode layers. By layers is meant that the foils and dielectric mechanical separators are arranged in the form of stacked plates, sheets, strips and the like. In a preferred embodiment a header is crimped or otherwise attached to an end of the casing, the header including a primary anode terminal means and/or a primary cathode terminal means.
In the method of the invention, a wound capacitor is formed by gripping a thin flexible foil cathode layer with a mandrel to start the winding process, sandwiching the cathode foil layer between mechanical separator layers, spiral winding the multilayered laminate, interleaving a solid core anode foil first layer between adjacent mechanical separator layers of the spiral winding which frictionally engage the anode foil layer and hold same in place, and progressively thereafter interleaving one or more porous anode foil layers between the solid core anode foil first layer and adjacent mechanical separator layers, with the mechanical dielectric separators arranged to maintain the cathode foil winding from physically contacting the anode foil windings.
It has been found, that such combination of windings of the method, enables the use of an extremely small diameter winding mandrel and can significantly reduce the open space occasioned when the mandrel is withdrawn at completion of the winding. Indeed it has been found that using this method of assembly, the primary limitation to down-sizing of the mandrel diameter is that it comprise sufficient strength to enable gripping the end of a flexible core cathode foil, with and/or without a dielectric separator layer, and support the accumulation of windings to the desired circumference of the capacitor.
The applicants have found that during spiral wrapping, commercially available thin flexible cathode foil and thin separator layers can generally be bent to extreme acute angles without breaking and that the incidence of breakage of these components is not a significant impediment to the use of small diameter mandrels. Indeed, the limitation to mandrel size which is appears to be imposed by these components is that the mandrel have sufficient mass to grip the winding and support it through the winding process. Thus, for example, for capacitors sized to be used in implantable defibrillators, a mandrel diameter of about 1.0 mm or less might be suitably used with such components.
Though porous anode foils are generally considered necessary components for obtaining the highest possible capacitor efficiency in a wound capacitor, commercially available porous anode foil is so brittle that there is a high incident of breakage of anode foils when they are bent to acute angles under tensile wrapping stress in situations wherein the circumferential wrap diameter is less than about 5.0 mm. Breakage of a porous anode during assembly disrupts the efficiency of the manufacturing process by creating a failure in the winding process, and also disrupts the continuity of the foil, decreasing the overall effective storage capacity that can be conveniently discharged through typical positioned terminal ends of a wound capacitor. Thus, the economics of manufacturing and the requirements of capacitance consistency appear to counteract any advantage which might be gained by reducing center space through down-sizing the mandrel diameter from about 5.0 mm in a wound capacitor comprising a porous anode.
Applicants have found however, that having one or more anode foils in engaged opposing layered arrangement, significantly reduces the probability of a break occurring at the same position in any two opposing layers, and that a disruption in the continuity in one opposing anode layer is generally bridged by the other opposing anode layer such that there is no significant loss in discharge surface area of the overall layered anode.
Further, applicants have found that solid-core anode foils have a significantly reduced tendency to break than porous anode foils and can be generally conveniently interleaved in a winding, without manufacturing process disrupting breakage, on a mandrel at circumferential winding diameters much less than about 5.0 mm. Indeed, applicants have interleaved solid core anode foils at circumferential winding diameters of 2.0 mm and even less without assembly disrupting breakage in capacitors sized for implantable defibrillators. Even if breakage occurs, it is generally limited to occurring at the extreme initial interleaved end of the anode after gripping of the anode between the separator layers is secure and therefore does not pose an assembly disrupting problem. Thus, if breakage of the solid core anode does occur, it generally only results in an inconsequential loss of capacitance corresponding to the insignificant quantity of continuity disrupted surface at the initial end of the wound solid core anode.
A single solid core anode foil, standing alone, is not generally considered a very high efficiency anode. To increase anode efficiency applicants have found it is preferred to use a porous anode and particularly to use a layered arrangement of porous anodes which enable access to inner anode surfaces for an acceptable internal resistance. Porous anodes are very brittle and tend to break in smaller sized capacitors used for implantable defibrillators, but if a solid core anode can be interleaved at a significant earlier point in a winding than is practical for interleaving a porous anode using current winding techniques, the additional length of interleaved solid core anode foil provides an increased total capacitance over the total capacitance which can be otherwise achieved using a porous anode.
Interestingly, applicants have found that brittle porous anode foil can be interleaved in a winding, without assembly process failure, at smaller turn diameters when interleaved between a separator layer and a solid core anode than when interleaved alone between two separator layers. It is speculated that even when a break occurs in the porous anode foil during winding, the irregular oxide comprising surface of the solid core anode engaging along the irregular oxide comprising surface of a porous anode foil provides sufficient grip to prevent assembly process failure. Thus, even though a continuity break of the brittle porous anode foil may occur, the assembly process is not disrupted and the broken porous anode foil remains in engaging relationship with a continuous surface of solid core anode foil.
The result is that interleaving a solid core anode foil layer first, provides a continuity base for attaining earlier interleaving of brittle porous anode foil than would be possible alone, and the final wound capacitor has more electrically continuous anode foil in opposed relationship to the cathode foil than can be otherwise achieved. Sandwiching the solid core anode foil with porous anode foil then provides a maximum capacitance continuous layered anode surface adjacent a cathode foil, which minimizes unused spacing in the winding and can be conveniently discharged at a remote end.
Thus, if the first anode interleaved is a solid core anode foil, with progressive interleaving of additional porous anode foils, mandrels of 2 millimeter diameter can be successfully employed. Such reduction in mandrel size enables an additional about six turns of interleaved anode to be employed opposite a cathode foil, in what otherwise would be open space, providing an approximate 8% gain in total available capacitance for a 12.7 mm diameter capacitor unit generally used in defibrillator devices.
In a preferred embodiment, the solid core cathode layer with dielectric separator layers sandwiched; thereover is initially wound through at least about one circumvention of the mandrel, followed by the interleaving of a solid core anode foil for the next about two circumventions of the mandrel, with one or more porous anode foils being thereafter progressively interleaved into the winding, juxtapositioned on either or both sides of the solid core anode foil, to form the desired winding arrangement.
In a particularly preferred arrangement of the invention, leads, commonly referred to as terminal strips, are attached to each of the anodes and the cathode foil, the leads comprising an attaching section which attaches to the foil and a projecting tab. Each of the attaching sections are arranged to be offset from the other attaching sections when the foils are wound together. The projecting tabs comprise the leads of the anode foils and are generally connected to a primary anode terminal. A lead of the cathode foil is generally connected to a primary cathode terminal. A suitable electrolyte permeates the wound capacitor body. The first anode foil has a first anode terminal strip affixed transversely to the first anode foil, the second anode foil has a second anode terminal strip affixed transversely to the second anode foil and is offset with respect to the first anode terminal strip. The third anode foil has a third anode terminal strip affixed transversely to the third anode foil, the third anode terminal strip being offset with respect to both the first anode terminal strip and the second anode terminal strip. The attachment of terminal strips and configuration of leads again acting to reduce the pretense of open spaces.
In a preferred embodiment, the dielectric separator layers are formed from a kraft capacitor dielectric tissue paper. In another preferred embodiment, each separator layer comprises two or more layers of kraft capacitor tissue paper.
The above features and advantages of the invention will become more apparent to those having skill in the art from the following written description, drawings and appended claims.